Thanks everyone for your helpful replies. I had already bought a pair of adjustable reach brake levers, and the 3-speed (not flat) handlebar, adjusted to keep her hands in as straight a line from the elbow to the grip as possible. I even cut the brake cable housings as short as was functional, and lubed the cables to reduce losses due to binding and housing flex. The next least expensive thing to change is the calipers and I wasn't sure what the best choice would be.
I'd seen the Tektros on the harris cyclery page, but wanted to learn other people's experiences to see if it would be worthwhile before I made the purchase.